While semiconductor integrated circuits are used in a variety of applications, a finer circuit design is demanded in order to increase an integration density and save power consumption. On the side of lithography technology using a photomask for circuit formation, the development of miniaturization technology is promoted from 45 nm node to 32 nm node and further to 20 nm node or less. To print images of finer size, a light source of shorter wavelength is used for exposure. In the current advanced commercial process, the light source has made a transition from KrF excimer laser light (248 nm) to ArF excimer laser light (193 nm). To print images of finer size, the super-resolution technology is advancing. For example, immersion lithography, modified illumination and supplement patterns are used. As to the photomask used in photolithography, there are developed not only binary photomasks consisting of a light transmitting section and a light-shielding section, but also phase shift photomasks utilizing optical interference including halftone phase shift masks, Levenson phase shift masks, and chromeless phase shift masks
The photomask blank which is a precursor to a photomask basically has the structure that inorganic films such as a light-shielding film for shielding exposure light and a phase shift film for changing the phase of exposure light are formed on a transparent substrate which is transmissive to exposure light. The photomask is prepared from the photomask blank by patterning the inorganic films such as light-shielding film and phase shift film thereon. As the inorganic film, for example, light-shielding film, a chromium base film containing chromium or a molybdenum silicon base film containing molybdenum and silicon is often used. Besides, a film of silicon alone is also known as a film capable of reducing EMF bias caused by electromagnetic field (EMF) effect.